90° 80° 70° 60° 50° 40° 30° 20° 10° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90°

CURVE OF PERPETUAL CONGELATION.

303. The heights of the curve from the surface of the globe vary but little from the equator to Lat. 30°; but from 30° to 60° the change is very great, and the line rapidly approaches the earth.

The difference in the height of the points of congelation, for every five degrees of latitude, is presented in the following table:

304. ACTION OF OPPOSITE CURRENTS. We are now to imagine, for the sake of illustration, that a north

Describe its peculiarities.

wind, originating in 50° N. Lat., moves horizontally at the rate of sixty miles per hour, at an altitude of ten thousand feet; while a south wind blows simultaneously from 30° N. Lat. with the like velocity, and at the same height.

If they are upon the same meridian, they will meet in ten hours at 40° N. Lat., and since the first wind commences its course at M, three thousand feet above the limit of constant frost, it will be extremely cold; while the south wind will be comparatively warm, as it proceeds from a region, N, two thousand feet below the boundary of perpetual congelation.

By the union of air, thus widely differing in temperature, the inherent atmospheric vapor is both condensed and frozen, and the central portion of the hailstone formed, which, in its descent to the earth, is gradually enlarged by constant accretions of frozen moisture.

305. The prevalence of such opposite currents as have just been supposed, has already been shown (Art. 222); and it is by no means improbable that, in their ceaseless circuits, there are times in which they encounter each other. It may be asked, how can the different winds preserve their respective temperatures, in traversing a distance of ten degrees? To this it is answered, that a fluid in motion can pass through a fluid of the same kind in repose, and differing in respect to heat, without suddenly changing its own temperature. The waters of the Gulf-stream, flowing through the North Atlantic from the torrid zone, are warmer than the ocean, even in the latitude of Newfoundland.

306. The occurrence of hailstorms, under these circumstances, substantially agrees with facts. It will be seen, by referring to the figure, that the mingling of opposite winds, at a lofty elevation, in the tropics, C, C2, would occasion nothing but a combination of warm cur

Explain the action of opposite currents.

Why can the currents preserve their respective temperatures?

Show to what extent the occurrence of hailstorms, under these circumstances, accords with facts.

rents, and in the polar climes of cold currents, A, A3; in neither case could hail be the result of the union.

In the temperate regions, the admixture of warm and intensely cold currents can only be found, and precisely within these limits hailstorms are most prevalent.

Their frequency in summer is attributed to the cir cumstance, that the opposing winds are then most easily set in motion by the increased energy of the solar

rays.

307. The space ravaged by hailstorms, often indicates the presence of aërial currents, the devastations being frequently confined to a long and narrow strip of country. Sometimes the storm proceeds in two parallel tracks, leaving the intervening region entirely uninjured.

Thus a hailstorm once commenced in the south of France in the morning, and in a few hours reached Holland. The places desolated formed two parallel paths from S. W. to N. E.; the length of one was 435 miles; and that of the other 497 miles. The average width of the eastern track was five miles, and that of the western ten; and upon the space comprised between them, which was twelve miles and a half in breadth, no hail fell, but only a heavy rain.

308. ACTION OF WHIRLWINDS. It has been stated, (Art. 132,) that whirlwinds are not always vertical, but frequently inclined towards the earth. In consequence of this position, the gyratory motion of the whirl (if its diameter is considerable) will, doubtless, often carry up hot and humid air from the surface of the earth into the higher regions of the atmosphere, bringing down in return large volumes of cold air from the upper strata; thus causing such a combination as results in the production of hail. This action will be more extensive and energetic if, as some suppose, whirlwinds at times exist whose axes are parallel to the horizon.

309. It must also be remembered, that in the vortez of the whirlwind the air is rarefied, and into this partial

Explain the action of whirlwinds.

void the cold air from above will descend, by reason of its superior weight; while below, on account of the. pressure of the surrounding atmosphere, warm currents will stream under the base into the vortex. Here, then, may evidently occur a union of hot and cold air, differ ing so greatly in temperature that the condensed moisture will freeze into hail.

The cold, arising from the rarefaction of the air in the centre of the whirlwind, also contributes to the formation of hail.

310. INFLUENCE OF HIGH MOUNTAINS. In the vici. ity of those lofty mountains, whose peaks are always covered with ice and snow, destructive hailstorms frequently occur. The south of France, which lies between the Alps and Pyrenees, is annually ravaged by hail; so great is the ruin to the productions of the soil, and especially the vineyards, that the yearly loss to the national revenue was estimated, by the Linnean Society of Paris, at fifty millions of francs, or nine millions three hundred and seventy-five thousand dollars.

In Peru, hail has been seen to fall; and on the 17th of August, 1830, it covered the streets of Mexico to the depth of several inches.

311. That such phenomena should arise in these and similar localities, is by no means surprising: for cold blasts of wind descending from the snowy summits of the neighboring mountains, and mingling with the warm air of the plains, could doubtless occasion these results; and the existence of such breezes is fully established.

312. HAIL IN SOUTHERN INDIA. Hail sometimes occurs within the tropics, even at a distance from those mountain-chains that rise above the limit of perpetual frost. Thus in India, in 16° 30' N. Lat., during the year 1825, hailstones fell at Darwar, of the size of pigeons' eggs; and in a similar storm, which happened at

In what localities do hailstorms occur?

Give instances.

What is the cause of hail in these regions?

Does hail ever occur at a distance from snow-capped mountains?

Trinconopoly, in 1805, the stones were as large as wal

nuts.

313. In conclusion, we may say in regard to this subject, that at present it is not fully understood. Much valuable information has been gathered, but hitherto no theory has been advanced, which completely accounts for all the facts that arise.